BUG/MINOR: http: Return an error in proxy mode when url2sa fails

BUG/MINOR: http: Return an error in proxy mode when url2sa fails

In proxy mode, the result of url2sa is never checked. So when the function fails
to resolve the destination server from the URL, we continue. Depending on the
internal state of the connection, we get different behaviours. With a newly
allocated connection, the field <addr.to> is not set. So we will get a HTTP
error. The status code is 503 instead of 400, but it's not really critical. But,
if it's a recycled connection, we will reuse the previous value of <addr.to>,
opening a connection on an unexpected server.

To fix the bug, we return an error when url2sa fails.

This patch should be backported in all version from 1.5.